Micro grid array solder interconnection structure for second level packaging joining a module and a printed circuit board

ABSTRACT

A solder interconnection between a module and printed circuit board or card is provided by a plurality of solder connections arranged in a micro grid array joining solder wettable pads on a major surface of the module to a corresponding set of solder wettable pads of the printed circuit board or card. The solder connections are column shaped with the height of each connection being at least about 1.4 times the diameter of the connection.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/035,538 filed Mar. 5, 1998 now U.S. Pat. No. 6,059,173.

TECHNICAL FIELD

The present invention relates to interconnection structures for joiningan electronic array module to a printed circuit board or card, and moreparticularly to a structure for forming solder interconnection jointsthat exhibit reduced stress resulting from thermal mismatch of themodule and the printed circuit board. The present invention isespecially suitable in providing micro ball grid arrays (μBGA). Thepresent invention is also concerned with the method for fabricating suchinterconnection structures.

BACKGROUND OF INVENTION

Micro ball grid array (μBGA) refers to a surface mounting packagingtechnique whereby the solder ball array pattern is typically less thanabout 0.75 mm pitch. This is contrasted to conventional ball grid arraytechnology whereby the pitch is at least about 1.0 mm. Pitch refers tothe distance measured from the center of an adjoining solder ball to thecenter of the next adjacent solder ball.

In second level packaging of an electronic array module or chip carrierto a printed circuit board or card, the solder interconnection is rigidand cannot flex under thermal mismatch conditions. As ball to ballspacing shrinks to 0.75 mm pitch and less, the diameter of the ball mustbe reduced to achieve such spacing. This results in a lower stand offbetween the module and printed circuit board or card, resulting in lessresilience to stress during expected thermal cycles due to the thermalmismatch between the module and the printed circuit board. Thereliability of such a second level package interconnect is thereforejeopardized due to the loss of resilience to stress. Accordingly, μBGAhas seen limited industrial applications because of reliabilityconcerns. Reliability can be addressed by employing epoxy compositionsto adhere the μBGA package to integrated circuit boards or cards inorder to minimize the effects of thermal mismatch between the device andthe circuit board or card. However, this is not especially satisfactorysince the epoxy compositions cannot be reworked in that they arethermosetting polymer materials.

SUMMARY OF INVENTION

An object of the present invention is to minimize the problem fromthermal mismatch between electronic array modules surface mounted on aprinted circuit board or printed circuit card. According to the presentinvention, the reliability is significantly increased. Moreparticularly, the present invention is concerned with a solderinterconnection for forming connection between a module or chip carrierand printed circuit board or printed circuit card that comprises aplurality of solder connections arranged in a micro area grid array thatjoins solder wettable pads on a major surface of the module to acorresponding set of solder wettable pads of the printed circuit boardor card. According to the present invention, the solder connections arecolumn shaped with the height of each connection being at least about1.4 times its diameter. By employing the required column shapedinterconnections specified by the present invention, the stand offdistance between the module and printed circuit board or card issignificantly increased. This in turn increases the resilience of theconnection during stress experienced during thermal cycles due to thethermal mismatch between the module and printed circuit board or card.

The present invention is also concerned with the method forinterconnecting a module to a printed circuit board or card whichcomprises attaching the circuit module to the printed circuit board orcard by a plurality of solder interconnections that extend from solderwettable pads on a major surface of the module to a corresponding set ofsolder wettable pads of the printed circuit board or card. The solderconnections are column shaped with the height of each of the connectionsbeing at least about 1.4 times its diameter and the solder connectionsare arranged in a micro area grid array.

Still other objects and advantages of the present invention will becomereadily apparent by those skilled in the art from the following detaileddescription, wherein it is shown and described only the preferredembodiments of the invention, simply by way of illustration of the bestmode contemplated of carrying out the invention. As will be realized theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects,without departing from the invention. Accordingly, the description is tobe regarded as illustrative in nature and not as restrictive.

DESCRIPTION OF DRAWING

The FIGURE is a schematic diagram of a solder interconnection pursuantto the present invention.

BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION

To facilitate an understanding of the present invention, reference ismade to the FIGURE. In the FIGURE, numeral 1 represents the electronicarray module or chip carrier joined to the printed circuit board orprinted circuit card 2 by the solder columns 3. The columns are arrangedin a micro grid array with a column pitch of 0.75 mm or less andpreferably about 0.5 mm to about 0.75 mm. Pitch refers to the distancefrom the center of an adjoining column to the center of its adjacentcolumn or columns. It is critical to the present invention that theheight of each column be at least about 1.4 times its diameter andtypically no greater than about 2.5 times its diameter, a preferredrange being about 1.5 to about 2.5 times its diameter. Some typicalcolumns according to the present invention are about 9 mils to about 13mils in diameter, and about 12.5 mils to about 33 mils in heightdepending upon the diameter. For example, the height for a 9 mildiameter column is typically about 12.5 to about 22.5 mils, and for a 13mil diameter column is typically about 18 to about 33 mils. In aspecific example, the diameter of the column can be about 12.5 mils withthe height being at least 17.5 mils and preferably about 31 mils. Inaddition, according to the present invention, each of the columns isapproximately the same height and substantially the same diameterthroughout the entire column.

It is desirable that the solder columns be non-collapsible attemperature used in fabricating the structures of the present invention.Typically, the column should be non-collapsible at temperatures of atleast about 220° C. A typical solder composition employed is an alloy ofabout 90% by weight lead, and about 10% by weight tin.

The solder is connected to a wettable contact pad (not shown) on themodule and to a corresponding solder wettable contact pad (not shown) onthe printed circuit board. Typically, the contact pads are copper. Themodules are ceramic or organic substrates that carry integrated circuitchips as is well known in the art. Typical ceramic carriers includesilicon oxides and silicates such as aluminum silicate, and aluminumoxides. The organic substrates can include thermoplastic and/orthermosetting resins. Many of such substrates contain the resin and areinforcing material such as fiberglass. Typical thermosetting resinsinclude FR-4 epoxy resins, phenolic based materials, BT resins andpolyimides. Examples of thermoplastic polymers include polyolefins suchas polypropylene, polysulfones, fluorocarbon polymers, polyethyleneterephthalate, polycarbonates, nitrile rubbers and ABS polymers.

The printed circuit boards or cards can be any of the printed circuitboards or cards well known in the art such as those from fiberreinforced epoxy resin compositions.

The foregoing description of the invention illustrates and describes thepresent invention. Additionally, the disclosure shows and describes onlythe preferred embodiments of the invention but, as mentioned above, itis to be understood that the invention is capable of use in variousother combinations, modifications, and environments and is capable ofchanges or modifications within the scope of the inventive concept asexpressed herein, commensurate with the above teachings and/or the skillor knowledge of the relevant art. The embodiments described hereinaboveare further intended to explain best modes known of practicing theinvention and to enable others skilled in the art to utilize theinvention in such, or other, embodiments and with the variousmodifications required by the particular applications or uses of theinvention. Accordingly, the description is not intended to limit theinvention to the form disclosed herein. Also, it is intended that theappended claims be construed to include alternative embodiments.

What is claimed is:
 1. A method for interconnecting a second level packaging of an electronic array module to a printed circuit board or card which comprises attaching said module to said board or card by providing a plurality of solder interconnections extending from solder wettable pads on a major surface of said module to a corresponding set of solder wettable pads of said board or card in a micro area grid array having a pitch of about 0.75 mm or less and wherein said solder interconnections are column shaped with the height of each being at least about 1.4 times its diameter; wherein said connections are about 9 mils to about 13 mils in diameter and about 12.5 mils to about 33 mils in height.
 2. The method of claim 1 wherein the pitch of the array of said connections is about 0.5 mm to about 0.75 mm.
 3. The method of claim 1 wherein said solder connections are non-collapsible at the temperature employed to fabricate the interconnection.
 4. The method of claim 3 wherein said temperature is at least about 220° C.
 5. The method of claim 1 wherein the height of said connections is up to about 2.5 times the diameter of said connections.
 6. The method of claim 1 wherein the height of said connections is about 1.5 to about 2.5 times the diameter of said connections.
 7. The method of claim 1 wherein each column is about the same height and substantially the same diameter throughout each column.
 8. The method of claim 1 wherein said columns are an alloy of about 90% by weight lead and about 10% by weight tin.
 9. The method of claim 1 wherein said module comprises a ceramic substrate.
 10. The method of claim 1 wherein said module comprises an organic polymer substrate.
 11. The method of claim 1 wherein said connections are about 12.5 mils in diameter and at least about 17.5 mils in height. 